Method of compensating for changes in flow characteristics of a dispensed fluid

ABSTRACT

A fluid dispensing control for controlling the dispensing of a fluid by a metering valve through a nozzle onto a workpiece. An initial value of a flow characteristic of the fluid is determined that is correlated to the relationship between the flow rate of the fluid and nozzle pressure. Desired nozzle pressure values are periodically determined by evaluating a model of flow rate of the fluid through nozzle in response to the initial value of the flow characteristic and a desired flow rate value. Thereafter, the control provides command signals to the metering valve as a function of the desired nozzle pressures. A new value of the flow characteristic is determined as a function of the measured volume of fluid dispensed during the dispensing cycle to the measured nozzle pressure. During a subsequent dispensing cycle, the control determines the desired nozzle pressures by evaluating the model of flow rate of the fluid through the nozzle as a function of the new value of the flow characteristic. The process of reevaluating the flow characteristic over successive dispensing cycles as a function of measured volumes of fluid dispensed and measured nozzle pressures, and using those updated values to reevaluate the model of flow rate of fluid through the nozzle, is repeated.

This application is a continuation application of U.S. Ser. No.08/961,840, entitled "Method of Compensating For Changes in FlowCristics of a Dispensed Fluid", filed Oct. 31, 1997, allowed which inturn is a continuation application of U.S. Ser. No. 08/435,972, filedMay 5, 1995, now U.S. Pat. No. 5,687,092.

FIELD OF THE INVENTION

The present invention relates generally to a system for dispensingfluids and, more particularly, the invention provides for a real timemethod of compensating for changes in the flow characteristics of thefluid being dispensed.

BACKGROUND OF THE INVENTION

The automated deposition of coating materials, such as adhesives,caulks, or sealants onto the surfaces of workpieces is commonlyperformed through the use of program control devices, such asrobot-mounted fluid dispensing guns. The devices which support the gunsare programmed to move the guns through a predetermined path withrespect to a workpiece surface which corresponds to a desired pattern ofapplication of the fluid onto the surface. In such devices, a controlprogram establishes the tool speed, while a fluid dispensing controlcontrols the discharge of fluid. The fluid is to be dispensed inaccordance with an operator defined input signal which defines a desiredphysical characteristic of the applied fluid. For example, the inputsignal may represent bead size which defines the desired diameter of thebead to be applied to the workpiece. To achieve the desired bead size,the rate at which fluid is dispensed from the gun nozzle must beproportional to the relative velocity between the workpiece and thedispensing gun. Therefore, the rate at which fluid is dispensed throughthe gun nozzle must vary proportionally in real time in response tochanges in the tool speed signal. The tool speed is defined as thelinear or scalar speed at which the point of application of coatingmaterial on the workpiece surface moves with respect to the workpiecesurface. The flow rate of fluid through the dispensing gun can becontrolled by measuring the pressure drop across the nozzle of thedispensing gun and controlling the operation of a metering valveregulating the flow of fluid through the gun.

The above fluid dispensing process is further subject to unpredictablechanges in the flow characteristics of the fluid being dispensed. Forexample, changes in temperature, and other conditions will change inreal time the flow characteristics of the fluid being dispensed; andthose changes in flow characteristics will change the flow rate andhence the volume of fluid dispensed. In addition, there are flownon-linearities introduced by the shear effects of the fluid flowthrough the dispensing nozzle; and those flow non-linearities aredependent on the nozzle and nozzle wear. Therefore, it is desirable thatthe volume of fluid dispensed over a dispensing cycle be a controlledvariable, and the total volume of fluid dispensed each dispensing cycleis measured.

As disclosed in the Baron, et al. U.S. Pat. No. 5,065,695 issued to theassignee of the present invention, the fluid dispensing controlcompensates the tool signal by a correction factor that is determined asa function of the changes in viscosity caused by shear effects of thefluid through the nozzle. As part of a setup calibration procedure, theflow of fluid through the nozzle is measured in response to differenttool speed signal settings thereby producing a table data values whichare stored in the fluid dispensing control memory. The stored data isused to calculate an interpolated linearization factor which is appliedto the adjusted tool speed signal. The stored linearization factor iscorrelated to the relationship between flow rate and nozzle pressure asmeasured during the calibration process. However, the stored dataremains fixed, and hence, the compensation is fixed over many dispensingcycles even though the relationship of flow rate to nozzle pressure maychange. While the change is compensated for in a volume measurementcontrol loop, the above system has the disadvantage of not being morequickly responsive to changes in the flow rate-nozzle pressurerelationship.

In addition, the volume of fluid measured during one dispensing cycle iscompared to a volume set point, and a material volume error signal isproduced that represents changes in material viscosity that are causedby temperature changes or other dynamic conditions. The material volumeerror signal provides a compensation for changes in material viscositythat are caused by temperature changes or other dynamic conditions. Thematerial volume error signal is produced from a proportional andintegrating comparator. The volume of material that is dispensed iscompared to a material weight setting, that is, a volume set point toproduce a material volume error signal. Within the proportional andintegrating comparator, a proportional term is set equal toapproximately one-half the error signal; and the integral term is equalto the difference between the proportional term and the prior integralterm. Consequently, the material volume error signal changes thepressure command signal gradually over several dispensing cycles tobring the volume of material that is being dispensed into conformitywith the volume set point. For example, five or more dispensing cyclesmay be required to effect the volume compensation. While the abovedescribed system performs the necessary compensation, a disadvantage ofthe system is that several dispensing cycles are executed before thecompensation is complete.

With the above system, the volume set point is determined by apreproduction experimental process in which a sample part is fixtured inthe proximity of the fluid dispensing nozzle to simulate a productionsituation. The dispensing cycle is then executed, and the fluiddispensing nozzle and the workpiece are moved relative to each othersuch that the fluid is applied to the sample part in the desiredpattern. Several dispensing cycles and parts may be required until thedispensed bead visually appears to be correct. When the correct bead isidentified, the volume flow meter for that particular dispensing cycleis read; and the value of the volume flow meter is utilized as thematerial volume set point. Thereafter, the volume set point is conveyedto the production environment as part of the fluid dispensing programassociated with that part.

There are several disadvantages with the above experimental process fordetermining the material volume set point. First, the experimentalprocess requires that fluid be dispensed on workpieces that mostprobably are not usable in subsequent production. In addition, thevolume set point is a part dependent parameter that must be carried withthe other part related information adds to the complexity and cost ofthe overall system. Second, the experimentally determined materialvolume set point is a function of the flow characteristics of the fluidbeing dispensed during the test cycle. The flow characteristics of thefluid being dispensed during a subsequent production cycle may bedifferent; and therefore, the volume represented by the previouslydetermined volume set point may require further compensation in theproduction environment.

SUMMARY OF THE INVENTION

To overcome the disadvantages described above, the present inventionprovides a fluid dispensing control requiring minimal calibrationprocedures which are automatically performed to provide an initial valueof a flow characteristic that is correlated to the relationship betweenthe fluid flow rate through the nozzle and nozzle pressure. The fluiddispensing control of the present invention periodically andautomatically determines a new value of the fluid flow characteristic inresponse to measured values of nozzle pressure and dispensed fluidvolume. The present invention further provides a fluid dispensingcontrol that is independent of the part or workpiece to which the fluidis being applied. For example, desired nozzle pressures within a fluiddispensing cycle are determined independent of a volume set point. Inaddition, the volume set point is determined automatically during adispensing cycle. The present invention provides a highly responsivereal time control of pressure and volume of the dispensed fluid that iseffective to provide a high quality application of the fluid.

According to the principles of the present invention and in accordancewith the described embodiments, a fluid dispensing control provides amethod of compensating for variations in fluid flow characteristics overmultiple dispensing cycles. The process determines initial values of aflow characteristic of the fluid correlated to the relationship betweenflow rate of the fluid through the nozzle and nozzle pressure.Thereafter, the control periodically determines during a firstdispensing cycle desired nozzle pressures by evaluating a model of flowrate of the fluid through the nozzle as a function of the initial valuesof the flow characteristic and a desired flow rate value. The controlperiodically provides command signals to the metering valve as afunction of the desired nozzle pressure values. In addition, thedispensing control measures during that first dispensing cycle thenozzle pressure and the volume of fluid dispensed; and thereafter, thecontrol determines a new value of the flow characteristic of the fluidthat correlates the measured volume of fluid dispensed during the firstdispensing cycle to the measured nozzle pressure. During a subsequentdispensing cycle, the control determines desired nozzle pressure valuesby evaluating the model of flow rate of the fluid through the nozzle asa function of the new value of the flow characteristic and a desiredflow rate value. During successive dispensing cycles, the control theniterates the process of determining new values of the flowcharacteristic as a function of the measured volume of fluid dispensedand measured nozzle pressure and reevaluating the model utilizing theupdated value of the flow characteristic. The above process has theadvantage of continuously in real time updating a flow characteristicfunction that is correlated to the relationship between the measuredflow rate of fluid and the measured nozzle pressure. Therefore, theinvention responds very quickly in real time to changes in the flowrate-nozzle pressure relationship which typically are caused by changesin temperature or other factors that change the viscosity of the fluidbeing dispensed. The invention therefore very quickly responds in realtime to changes in the fluid flow characteristics with the advantages ofproviding more consistency in the dispensing and application of thefluid, thereby providing a higher quality fluid dispensing process.

In another embodiment of the invention, the dispensing controlperiodically determines desired nozzle pressure values by evaluating amodel of flow rate of the fluid through the nozzle that is independentof a fluid volume set point. The above fluid dispensing control processhas a first advantage of operating independently the volume set pointwhich reduces the complexity of the control. Further, in another aspectof the invention, the volume set point is automatically determinedduring the dispensing cycle by integrating the desired value of the flowrate of the fluid over the dispensing cycle. Therefore, the prior volumeset point calibration cycle is eliminated with the advantage ofsimplifying the control and the process and eliminating the handling ofthe volume set point in association with the workpiece program.Therefore, the above process has the advantage of being substantiallymore efficient and easier to use.

In a further embodiment of the invention, the fluid dispensing controlmeasures the temperature of the fluid being dispensed from the nozzleand periodically evaluates a model of flow rate of the fluid through thenozzle that is a function of fluid temperature change, a desired flowrate value and an initial value of the fluid flow characteristic throughthe nozzle. This embodiment of the invention is particularly useful inthose applications where the temperature of the fluid being dispensedchanges within a dispensing cycle. Such conditions may be encountered ina particularly harsh manufacturing environments or in applying fluid toparticularly large parts. This embodiment provides an additional elementof real time control which compensates for environmental conditions thatproduces irregular and unpredictable changes in the fluid flowcharacteristics. Therefore, the above fluid dispensing control has theadvantage of being more sensitive and more responsive to changes in thefluid flow characteristics and thereby providing a higher quality andmore accurate dispensing and application of the fluid onto theworkpiece.

The fluid dispensing control of the present invention provides a fluiddispensing control that minimizes the amount of preproductioncalibration, that more quickly adjusts fluid flow in response to changesin the flow characteristics of the fluid, for example, viscosity, andthat adjusts fluid flow in response to detected changes in temperature.These and other objects and advantages of the present invention willbecome more readily apparent during the following detailed descriptiontogether with the drawings herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a fluid dispensing systemembodying the principles of the present invention.

FIG. 2 is a flow chart illustrating the general cycle of operation ofthe servo control microprocessor within the fluid dispensing control.

FIG. 3 is a flow chart of a subroutine of FIG. 2 illustrating thedetails of the servo control process of providing an output signal tothe servo valve.

FIG. 4 is a flow chart illustrating the process steps in the calibrationcycle of the fluid dispensing process.

FIG. 5 is a flow chart illustrating a process for dispensing fluid inaccordance with the principles of the present invention.

FIG. 6 is a flow chart illustrating a process for evaluating theconstants representing the flow characteristics of the fluid through thenozzle.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a fluid dispensing system 20 comprising a fluiddispensing control 22 which provides an output command signal on line 24to an electromechanical servo actuator 26 operatively connected to afluid dispensing gun 28. The servo actuator 26 may be any one of anumber of different types of linear actuators having a rapid response.The servo actuator 26 preferably includes an electrically actuatedpneumatic servo valve 27 that ports air to a double-acting pneumaticcylinder 29. The cylinder 29 has a cylinder rod (not shown) mechanicallycoupled to a stem (not shown) of a metering valve 30 within the fluiddispensing gun 28. The fluid dispensing control 22 provides outputsignals to the servo actuator 26 which controls the operation of themetering valve 30, thereby regulating the flow of fluid from a reservoiror source of fluid 32 through an orifice 34 of nozzle 36. Preferably thefluid dispensing gun 28 dispenses a bead 37 of fluid, for example, acaulk, sealant or adhesive onto a workpiece 38 moving with respect tothe fluid dispensing gun 28. Typically, either the fluid dispensing gun28 and/or the workpiece 38 are mounted on a robot arm, moving table, orother device which has a control independent of the fluid dispensingcontrol 22.

The fluid dispensing control 22 includes operator I/O devices 40 whichpreferably include a keypad and video display terminal (not shown).

The operator I/O devices 40 further include a processor (not shown) forhandling the communication of data between the video display terminaland the keypad and the serial I/O 42. The processor associated with theoperator I/O devices 40 may be any one of a number of commerciallyavailable processors or PC's. Preferably, the processor within theoperator I/O devices 40 is a "NEURON CHIP" 3150 processor commerciallyavailable from Motorola of Phoenix, Ariz. The processor in the operatorI/O devices 40 is connected by a twisted pair cable 41 to the operatorI/O interface 44 within the serial I/O interface 42. Preferably theoperator I/O interface 44 is also a "NEURON CHIP" 3150 processor andfunctions to exchange data with the bus interface 46. The bus interface46 handles the transfer of data to and from a bus 48 which is preferablya parallel 16 bit data bus. Although not shown, the serial I/O 42 mayalso include other serial communications links, for example, other"NEURON CHIP" processors, an RS-232 port, etc.

The bus 48 is connected to a supervisor control 49 which includes acentral processing unit "CPU" 50, preferably, a 68000 microprocessoravailable from Motorola of Phoenix, Ariz., volatile and nonvolatile RAMstorage 52, ROM storage 54, and bus 55 connected to a bus interface 56.The supervisor control 49 functions basically as an I/O processor andcoordinates the communication of data to and from the operator I/Odevices 40 and to and from external devices 57 generally located remotefrom the fluid dispensing control 22. Since the general modes and cyclesof operation are initiated either by inputs from the operator I/Odevices 40 or the external devices 57, the supervisor control 49provides input signal states to the servo control 74 which executesvarious tasks in the fluid dispensing cycle. The operator I/O devices 40is used to initiate different modes of operation, for example, a set upmode, and an operating mode. In the set up mode, the operator uses theI/O devices 40 to enter information relating to the desired flow, forexample, bead size, and scaling factors such as the flow meter encoderpulse count per revolution.

In addition, the operator I/O devices 40 display information relating tothe dispensing process, for example, alarm or error signals.

The supervisor control 49 stores various operating programs in ROM 54that command desired sequences of tasks or events depending on thedesired current control operation and detected external conditions. Aswill be described in more detail, the supervisor control 49 providescommands within the fluid dispensing control that start and end adispensing cycle, that turn ON and turn OFF the dispensing gun, etc. Thesupervisor control 49 provides other schedules of events depending onthe then current operation of the fluid dispensing control 22.

The fluid dispensing control 22 further includes digital I/O 58 whichhas a digital I/O interface 60 that provides and receives digitalsignals to and from, respectively, external devices 57 within the fluiddispensing system. Preferably, the digital I/O interface 60 provides 16bits of I/O data.

Input data typically includes beginning of part and end of part signals,a part identification word, a dispensing gun ON/OFF signal, etc. Inputsignals are received from the external devices 57 that may or may nothave their own respective digital I/O interface (not shown) on one ofthe digital I/O lines 62 connected to a respective input of the digitalI/O interface 60. Those input signals are passed to the bus 48 by a businterface 64, and the supervisor control 49 receives the input signalsfrom the bus 48 through its bus interface 56. During its operation, thesupervisor control 49 will detect different conditions and processstates. Those process conditions include the values of measured processvariables, for example, nozzle pressure, material temperature, and errorconditions; and the supervisor control will either provide some of thoseprocess conditions to the display within the operator I/O devices 40, orprovide output digital signals representing those process conditionsfrom the fluid dispensing control 22, or provide both. In the case ofproviding a digital output signal, the CPU 50 within the supervisorcontrol 49 will transfer the digital output signal to the bus interface56, across the bus 48, to the bus interface 64 and to a respectiveoutput of the digital I/O interface 60. That digital output signal isthen available on a respective one of the digital I/O lines 62 and isread or received by the external devices 47 within the system.

The fluid dispensing control 22 further includes a servo control 74operating in conjunction with the supervisor control 49. Data isexchanged between the servo control 74 and the supervisor control 49through a dual port RAM 70. The dual port RAM 70 is preferably a 16 bitshared memory device commercially available from Cypress of San Jose,Calif. Within the servo control 74, a microprocessor 76 executesprograms or routines stored in ROM 80. In executing those programs, themicroprocessor utilizes the RAM 78, floating point math coprocessor 82,and an interconnecting 16 bit parallel bus 83. The microprocessor 76 ispreferably a model 68HC16 microprocessor, and the coprocessor 82 is alsopreferably one of the 68000 family of processors commercially availablefrom Motorola of Phoenix, Ariz. Analog data is received from variouscomponents within the fluid dispensing system 20 and is converted tocorresponding digital signals by an A/D converter 88 which preferably isa 10 bit A/D converter available on the microprocessor 76.

Upon power being applied to the microprocessor 76 and other deviceswithin the servo control 74, a power ON or reset program or routinestored in ROM 80 is executed. Referring to FIG. 2, the power on routineat 200 first executes initialization and self test subroutines. Thosesubroutines run standard tests of the hardware within the servo control74. The remainder of the power ON routine is a real time task schedulerwhich preferably responds to a 2 millisecond (ms) clock. The power ONroutine at 202 first initializes the task scheduler. Initializationincludes resetting the counters and/or timers which are included withinthe scheduler and, if necessary, priorities of the scheduled tasks arerearranged.

The power ON routine continues at 204 to update the task timer withinthe task scheduler. Thereafter, at 206 the routine determines whether torun 2 ms tasks, for example, the servo control subroutine at 208 withinthe power ON routine. The operation of the servo control subroutineshown in FIG. 3, controls the dispensing of fluid from the nozzle andwill be subsequently described. The power ON subroutine at 209 thendetermines whether the tasks which are iterated every 10 ms should berun. If so, the routine executes, for example, the flow controlsubroutine at 210, which is illustrated in more detail in FIG. 5 andwill be subsequently described. The routine then at 212 determineswhether the flow control cycle end, that is, an end of part or end ofdispensing cycle, has been detected; and, if so, the routine causes themost recently determined IPN and flow counter values to be written tothe appropriate locations in the dual port RAM 70. The routine thendetermines at 216 whether it is time to run the diagnostics subroutineat 218 which is executed every 250 ms. Thereafter, the routine returnsto update the task timer at 204; and the microprocessor 76 continuouslyiterates through process steps 204 through 218 for as long as power isapplied to the servo control 74.

The servo control subroutine at 208 of FIG. 2 is illustrated in detailin FIG. 3. With each execution of the servo control subroutine 208, thesubroutine first at 220 reads the desired pressure which is determinedby the servo control 74 and stored in a location within the RAM 78. Ifthe desired pressure is zero as detected at 222, the microprocessor thenreads a minimum current value 224 stored in ROM 80 and outputs at 226that minimum current value to D/A converter 116. The analog output fromthe D/A converter 116 is amplified in servo amp 118 and a minimumcurrent command signal is provided on output line 24 to the servo valve26. Thereafter, the microprocessor at 228 samples the analog pressuresignal provided by the pressure transducer interface 96 of FIG. 1. Thepressure transducer interface 96 preferably includes a high impedanceinput instrument amplifier with good noise rejection characteristics. Alow pass filter is also used to stabilize the pressure signal on line 98from pressure transducer 100. Pressure transducer 100 is mounted on thedispensing head 28 close to the dispensing nozzle 36 and measures thepressure drop of the fluid as it is discharged through the orifice 34.The microprocessor 76 reads and stores a digital pressure signal fromthe A/D converter 88 as provided by the pressure transducer interface96. Thereafter, the microprocessor 76 at 230 integrates nozzle pressurevalues that will subsequently be described.

At 232 the microprocessor 76 samples an analog temperature valueprovided by the RTD interface 102. The RTD interface 102 is a resistancetemperature device that receives a temperature signal on input line 104from a temperature sensor or transducer 106. The temperature sensor 106measures the temperature at a point in the fluid stream that is upstreamand ahead of, but in proximity to, the dispensing gun 28. Preferably,the temperature transducer 106 is a standard nickel temperature sensorhaving a 120 OHM and is commercially available from Minco ofMinneapolis, Minn. The microprocessor 76 reads and stores a digitaltemperature value from the A/D converter 88 as provided by the RTDinterface 102.

At 234 the microprocessor reads and stores a digital stem velocitysignal from A/D converter 88 as provided by the velocity interface 94.The velocity interface receives an analog stem velocity signal on itsinput line 92 from a velocity transducer (not shown) which is mounted onthe dispensing gun 28. The transducer is mounted with respect to themetering valve such that the velocity transducer provides a velocityfeedback signal as a function of motion of the stem of the meteringvalve.

The microprocessor then at 236, 240 of FIG. 3 uses the stem velocityfeedback signal and the pressure signal to execute digital PIDprocesses. Proportional, integral and derivative terms are calculatedfrom the pressure signal; and a proportional velocity term is calculatedfrom the velocity signal. Each of those terms has a gain or multiplierthat is in the range of from zero to a value and that is empiricallydetermined to provide the desired response and stability to theoperation of the servo actuator 26 and the metering valve 30. Forexample, the gain of the proportional and derivative terms of thepressure signal is reduced to one-third its normal value when thedispensing gun is initially turned ON. Thereafter, the gain value forthose proportional and derivative terms of the pressure signal isgradually increased with time to a its normal value. Thereafter, at 242the microprocessor 76 determines whether the desired current value isgreater than a predetermined maximum value. If so, the microprocessor 76at 244 sets current value to the predetermined maximum current value.Similarly, at 246 the microprocessor 76 determines whether the outputcurrent value is less than a predetermined minimum; and if so, at 248sets the output current value at 248 to be equal to the predeterminedminimum value. As previously indicated, the output current value isconverted by the digital/analog converter 116 at 226 to a desired analogvalue, is amplified by the servo amplifier 118 and is output on line 24to the servo valve 26.

To provide effective fluid dispensing control, the servo control 74periodically evaluates during each dispensing cycle a model of flow rateof fluid through the nozzle to periodically determine a desired nozzlepressure as a function of the desired flow rate during a dispensingcycle. The model of flow rate of the fluid through the nozzle isexpressed as follows.

    FR=A×P.sup.N ×e.sup.b×ΔT

where

FR=Adhesive Flow Rate

P=Nozzle Pressure

A=First Flow Characteristic Constant

N=Second Flow Characteristic Constant

b=Temperature Sensitivity Factor

Δt=T_(i) -T_(i-1)

In shorter dispensing cycles it can be reasonably assumed that thetemperature will remain relatively constant. Therefore, in those cycles,the change in temperature can be assumed to be zero; and the model maybe simplified as follows.

    FR=A×P.sup.N

The model requires that initial valves be established for the terms orconstants A and N. Those initial values may be established by running amaterial calibration cycle or flow characteristic calibration process.The A term represents a flow characteristic constant that is correlatedto the relationship between the flow rate of fluid through the nozzleand nozzle pressure. Therefore, the value of A will be dependent on theviscosity of the fluid. Further, the value of the A takes into effectthe flow non-linearities that result from the shear effects from a givennozzle. The value of the N term is correlated to and is more directlyinfluenced by the flow non-linearities caused by shear effects of thefluid flow through the nozzle. Therefore, preferably anytime a nozzle ischanged or anytime the type of fluid being dispensed is changed, thevalues of the constants A and N should be reevaluated by running thematerial calibration cycle.

Thereafter, to the extent there are any environmental factors thatchange the fluid flow characteristics, a periodic recomputation of thevalue of A will reflect the change in the relationship between a flowrate of fluid from the nozzle and nozzle pressure that is caused bythose changes in fluid flow characteristics. Since one object of thepresent control is to provide a fluid dispensing control that isindependent of the particular workpiece or part, the materialcalibration cycle is executed by simply purging the desired fluidthrough the nozzle to be used in production with instrumentation thatpermits the detection of a measured volume of material dispensed and ameasured nozzle pressure. Preferably, the fluid dispensing control 22 isused to automatically run the material calibration cycle. The materialcalibration cycle is selected and started by an operator using theoperator I/O devices 40. Upon the start of the calibration cycle, thesupervisor control 49 initiates a sequence of events which are generallyillustrated in the subroutine of FIG. 4.

In running the subroutine, the supervisor control 49 first at 300provides a maximum desired pressure value to the dual port RAM 70 of theservo control 74 to dispense or purge fluid through the nozzle 36 withthe metering valve 30 in the dispensing gun 28 open to its maximum, thatis, full open position. In response to that maximum desired pressure,the microprocessor 76 executing the subroutine of FIG. 3 produces anoutput signal via the D/A converter 116, servo amp 118 and line 24 tothe servo valve 26 to operate the servo valve 26 and the metering valve30 so that fluid is dispensed from the dispensing gun at maximum, thatis, 100% of the nozzle pressure. The microprocessor 76 is also readingthe value of the velocity signal on line 92 and the pressure signal online 98. Pursuant to the servo control subroutine of FIG. 3, themicroprocessor 76 performs a digital PID processing of the pressure andvelocity feedback signals to maintain the output current signal on line24 at a value commensurate with the maximum desired pressure signal.

During the dispensing period, as previously described with regard toFIGS. 2 and 3, the microprocessor 76 periodically reads and stores ameasured pressure from the pressure transducer 100. At the time thesupervisor control 49 provides the maximum desired pressure value to theservo control 74, it also provides a reset counter command to cause theservo control 74 to reset the counter 114. Simultaneously therewith, thesupervisor control 49 starts a cycle timer that measures the time periodof the calibration fluid dispensing cycle. During the dispensing cycle,the supervisor control 49 periodically, for example, every 10 ms,provides a read counter command to the dual port RAM 70 requesting themicroprocessor 76 to read the counter 114 and write that value in arespective location in the dual port RAM 70. When the counter 114reaches a predetermined count, for example, 1000, which count isdetected by the supervisor control 49, the supervisor control 49 thenstops the operation of the cycle timer and reads and stores the elapsedtime measured by the cycle timer. The supervisor control 49 thenproduces a zero value desired pressure command to the servo control 74that is effective to turn the dispensing gun OFF. During the above 100%pressure calibration cycle, the supervisor control 49 also periodicallyreads from the dual port RAM 70 a measured nozzle pressure detected bythe servo control 74. Preferably, the supervisor control 49 calculatesan average of those pressure values and stores the average pressurevalue for future use. Therefore, at the end of the 100% pressurecalibration cycle, the supervisor control 49 has stored a dispensedvolume value, a measured nozzle pressure value and a dispensing cycletime period.

The supervisor control 49 also attempts to detect a nozzle clog or someother event that prevents the dispensing of the desired volume of fluid.For example, if the 1000 pulses are not counted within a predeterminedperiod, for example, a 90 second period, the supervisor control 49 endsthe 100% pressure calibration dispensing cycle and produces acalibration error message to the operator I/O devices 40.

A similar process is repeated at process steps 302, 304 and 306 in whichthe fluid volume and nozzle pressure are measured and stored fordispensing cycles which are executed at desired pressures equal to 50%maximum pressure, 25% maximum pressure, and 10% maximum nozzle pressurevalues, respectively. However, since lesser volumes of fluid aredispensed with successively lower pressures, the number of pulsesdetected during each of those cycles is successively smaller. Forexample, in the successive calibration cycles at 50%, 25% and 10%maximum nozzle pressure, the dispensing cycle is ended after the counter114 has detected 600 pulses, 300 pulses and 100 pulses, respectively.Similarly, absent those numbers of pulses being counted, the supervisorcontrol 49 ends the successive calibration cycles at 50%, 25% and 10%nozzle pressure after 90 seconds, 120 seconds and 240 seconds,respectively; and provides respective calibration error messages.

The purpose of the material calibration process is to determine the thencurrent fluid flow characteristics for the fluid and nozzle being used.It is known that the relationship of pressure to volume flow or flowrate is nonlinear; and therefore, in determining those characteristics,the supervisor control 49 uses the natural log values of the measureddata to approximate the best linear relationship that can be associatedwith the measured data. The supervisor control at 308 computes fournatural log of pressure values as follows:

    X.sub.n =In P.sub.MEAS @X%P

where

P_(MEAS) @X%P =Average Measured Pressure at the Set % of MaximumPressure

In addition, the supervisor control computes four natural log of flowrate values for each of the four calibration dispensing cycles asfollows: ##EQU1## where VOL_(MEAS) @X% =Measured Volume at the Set % ofMaximum Pressure

Two dimensional data coordinate values are defined by each of the fournatural log of pressure values and a corresponding one of the fournatural log of flow rate values. The supervisor control 49 at 310performs a linear regression on the four sets of coordinate values, forexample, a least squares regression, to identify a straight linerepresented by the four data points computed at 308. Next, at 312 thesupervisor control 49 sets a value of the constant N equal to the slopeof the straight line identified at 310 as follows: ##EQU2## Thesupervisor control 49 also determines a calibration value for theconstant A as follows:

    A=ε.sup.y intercept

The supervisor control 49 also at 312 stores the calibrated values ofthe constants A and N in RAM 52, and the control 22 is ready to begin aproduction cycle. The above material calibration process provides acalibration of the material flow and the nozzle at a time immediatepreceding production, and therefore, is more accurate than priorcalibration processes. The process is preferably performed at fourdesired pressures but may be performed at three desired pressures, andtherefore, is easier and less complex than prior methods. Further, thematerial calibration may be done automatically by purging fluid throughthe gun, and no workpieces or parts are needed.

After the material calibration cycle is completed, the fluid dispensingcontrol is switched by the operator via the I/O devices 40 to anoperating or fluid dispensing mode, the process of which is executed bythe flow control subroutine at 210 of FIG. 2. The flow controlsubroutine at 210 is stored in ROM 80 of the servo control 74, and thesubroutine at 210 is illustrated in more detail by the flow chart ofFIG. 5. Immediately upon the operator initiating the operating mode, thesupervisor control 49 awaits the receipt of a cycle start signal on oneof the digital I/O signal lines 62. When the workpiece 38 and dispensinggun 28 achieve a predetermined relative position, that predeterminedposition is detected; and a part strobe signal that is, an ON state, isprovided on one of the digital I/O input lines 62 as a cycle startsignal which is detected by the supervisor control 49 and is written tothe dual port RAM 70. The cycle start signal is read from the RAM 70 bythe servo control 74 at 350. After detecting a cycle start signal at350, the servo control 74 then reads the values of A and N from the dualport RAM 70. The values of A and N were previously updated to the dualport RAM 70 by the supervisor control 49 which stores those values inthe nonvolatile RAM 52. Thereafter, at 352 the servo control 74 sets thecurrent value of the desired volume to zero and resets the value of IPNto zero. The IPN and desired volume values are determined by a digitalintegration process over the dispensing cycle and therefore those valuesare set to zero at the beginning of the dispensing cycle.

Thereafter, an ON state of a dispensing gun ON/OFF signal is provided toanother of the digital I/O input lines 62 which is detected by thesupervisor control 49, is written to the dual port RAM 70 and isdetected by the microprocessor 76 within the servo control 74. The servocontrol 74 then reads the bead size value and the current value of thetool speed at 358. The bead size value is a constant entered by theoperator that represents the desired diameter of the bead of fluid to beapplied to the workpiece. The bead size is stored within the nonvolatileRAM 52 of the supervisor control 49, and the supervisor control 49writes the bead size value into the appropriate location of the dualport RAM 70 for use by the servo control 74.

The tool speed value represents the relative velocity between theworkpiece 38 and the dispensing gun 28. As previously noted, either orboth may be moving depending on the application. A tool speed signal isreceived on line 84 and processed by a tool speed signal conditioningcircuit 86. Preferably the tool speed signal conditioning circuit 86 hasa common mode noise rejection input configuration to provide noiseisolation, and further includes a low pass filter to provide a stabletool speed signal of the proper voltage level to the A/D converter 88.Therefore, the microprocessor 76 samples the output of the tool speedsignal interface 86 and writes the digital tool speed signal value fromA/D converter 88 into an appropriate location within the dual port RAM70.

At 360 the servo control 74 computes the desired flow rate which is theproduct of the values of the tool speed, the bead size and a scalingfactor which scales the magnitude of the desired flow rate value so thatit is compatible with the scale of other dimensional units.

    FR.sub.DES =BS×TS×k

where

BS=Current Value of Bead Size

TS=Current Value of the Tool Speed Signal

k=Scaling Factor

At 362, the servo control 74 then computes the desired volume or volumeset point of the fluid to be dispensed during the dispensing cycle. Incontrast to prior fluid dispensing controls, the volume set point is notutilized in determining the desired nozzle pressure; but with thepresent dispensing control, the volume set point is utilized betweendispensing cycles to determine any error between the measured volume offluid dispensed and the desired volume, that is, the volume set point. Aunique aspect of the present invention is that the volume set point isdetermined in real time by integrating the value of the desired flowrate over the dispensing cycle. During a particular sample period, forexample, a 10 ms period, the desired flow rate will theoretically resultin the desired volume of fluid being dispensed over that sample period.The integration, that is the accumulation or summation, of the desiredflow rates for each of the sample periods represents in real time thetotal desired volume of fluid to be dispensed, that is, the volume setpoint. Therefore, at 362 the servo control 74 performs a step of thatintegration as follows:

    VOL.sub.DES =VOL.sub.DES +FR.sub.DES ×0.01

For a sample period of 10 ms, the servo control 74 multiplies the valueof the desired flow rate by 0.01 and adds that value to a storedaccumulation of integrated desired flow rate values from previousexecutions of the step 362 during the dispensing cycle. Therefore, atthe end of the fluid dispensing cycle, the accumulated desired volumeprovided by the process at step 362 represents the volume set point forthat dispensing cycle.

Thereafter, if the temperature compensation has not been selected asdetected at 364, the servo control 74 at 366 computes the desired nozzlepressure in accordance with the following. ##EQU3## where P_(DES)=Desired Nozzle Pressure

FR_(DES) =Desired Flow Rate

A=First Flow Characteristic Constant

N Second Flow Characteristic Constant

The above is derived by rearranging the flow rate model previously setforth. The servo control 74 updates the new desired nozzle pressurevalue in the RAM 78 within the servo control 74; and flow of fluidthrough the nozzle is then commanded by the servo control 74 inaccordance with new nozzle pressure value pursuant to the servo controlroutine of FIG. 3 previously described. The flow control process at 356checks whether the fluid dispensing gun ON/OFF signal is still ON. Aslong as the dispensing gun ON/OFF signal remains in its ON state, theservo control 74 iterates through dispensing cycle steps 358-366 on aperiodic basis, for example, every 10 milliseconds, as determined by thepower ON routine of FIG. 2. The dispensing cycle is terminated by thedispensing gun ON/OFF signal switching to an OFF state which is detectedby the servo control 74 at 356; and the process returns to detect an endof cycle at 212 of FIG. 2. During a dispensing cycle, the dispensing gunmay be turned ON and OFF several times depending on the nature of theworkpiece, the pattern of fluid to be dispensed, etc. Even though adispensing gun OFF state is detected, if an end of flow control cycle,that is an end of part, is not detected by the servo control 74 at 212of FIG. 2, the process of FIG. 5 will await the occurrence of anotherdispensing gun ON signal. When that signal is received, the flow controlcycle of FIG. 5 will be executed every 10 ms as described above.

At the end of a part, that is, an end of a flow control cycle, the partstrobe signal changes to an OFF state on the digital I/O lines 62 andthat change of state is detected by the supervisor control 49 whichwrites that OFF state to the dual port RAM 70. The end of flow controlcycle is detected at 212 of FIG. 2, and the servo control 74 writes thenew values of the IPN and material flow volume into the dual port RAM70. The derivation and use of the IPN value will subsequently bedescribed. The material flow of volume is represented by the value thataccumulates in the counter 114 over a dispensing cycle. The counter 114is connected to a pulse encoder 110 that produces pulses on line 112that are detected and counted by the counter 114. The pulse encoder 110may be any device that produces pulses in a range from approximately 100pulses per revolution to approximately 2,000 pulses per revolution of aninput shaft. The choice of a particular encoder is generally dependenton the application and other components used in the dispensing system.The input shaft of the pulse encoder 110 is connected to a gear-typeflow meter 108 which is operated by fluid flowing from the fluid source32 to the dispensing gun 28. Therefore, during the dispensing cycle thecounter 114 maintains a pulse count that is proportional to the detectedor measured volume of fluid flowing through the dispensing gun 28 asmeasured by the flow meter 108.

When the part strobe signal changes to an OFF state as detected by thesupervisor control 49, the supervisor control then proceeds to computenew values for the flow characteristic constants as illustrated in thesubroutine of FIG. 6. Before recomputing any of the flow materialparameters, the subroutine first determines whether, over the priordispensing cycle, the volume of fluid dispensed exceeded a predeterminedminimum value, and further, whether the average nozzle pressure duringthat cycle exceeded a predetermined minimum pressure value. Therelationship between flow rate and pressure is an exponential one andhas a graphical representation that is generally parabolic in shape.Therefore, pressure changes at lower pressure values produce very littlechange in flow rate. The values of the minimums chosen will be afunction of the desired accuracy of the system and the resolution of themeasuring system, that is, the resolution of the pulse encoder 110 andthe flow meter 108. The minimums must be chosen so that a recomputationof the flow characteristic constants will increase the accuracy of thefluid dispensing process and not introduce instabilities therein.Therefore, the supervisor control 49 at 400 of FIG. 6 determines whetherthe volume of fluid dispensed during the previous cycle as measured bythe encoder 110 and counter 114 is greater than a predetermined minimumvolume. In addition, at 402 the supervisor control 49 determines whetherthe IPN value which is correlated to the average pressure during theprior dispensing cycle is greater than a predetermined minimum IPNvalue. If either one of those conditions is not met, the subroutine ofFIG. 6 is terminated. If both conditions are met, the supervisor control49 then recomputes at 404 the value of the flow characteristic constantA.

The value of A is correlated to the relationship between measured flowrate of fluid through the nozzle and nozzle pressure. Consequently, ifthere has been a change in temperature which changes the viscosity ofthe fluid, that in turn, modifies the relationship between the flow rateof fluid through the nozzle fluid an nozzle pressure, and that change inviscosity will be accommodated in the recalculation of the flowcharacteristic constant A as follows: ##EQU4## where VOL_(MEAS)=Measured Volume of Fluid Dispensed

k=Scaling Factor

The value of IPN is determined as follows: ##EQU5## where P_(MEAS)=Measured Pressure During Dispensing Cycle

N=Second Flow Characteristic Constant

start=Start of Dispensing Cycle

end=End of Dispensing Cycle

The integration of (P_(MEAS))^(N) is performed in the servo controlsubroutine of FIG. 3 as part of the integrate nozzle pressure valuessubroutine at 230. During each iteration through the servo controlsubroutine which occurs every 2 ms, the integration of (P_(MEAS))^(N) isperformed as follows:

    IPN=IPN+(P.sub.MEAS).sup.N ×0.002

Therefore, at the end of a dispensing cycle, the servo control processof FIG. 3 has at step 230 computed the IPN value. That value at 212 ofFIG. 2 is written into a respective location of the dual port RAM 70 sothat it is available to the supervisor control 49 for its recalculationof the flow characteristic constant A.

The supervisor control 49 thereafter at 406 computes the percentageerror between the desired volume of fluid to be dispensed and themeasured volume of fluid dispensed. The desired volume of fluid, thatis, the volume set point, was determined by the servo control 74integrating the desired flow rate over the period of the past dispensingcycle as discussed in FIG. 5 at 362. The volume error is computed inaccordance with the following. ##EQU6## where VOL_(ERROR) =Percent Errorin Volume of Fluid Dispensed

VOL_(DES) =Desired Volume of Fluid to be Dispensed

VOL_(MEAS) =Measured Volume of Fluid Dispensed

Thereafter, at 408 the volume error value is compared to a maximumvolume error value. The maximum volume error value is typically chosenas a function of the accuracy to which the volume is to be controlledwith respect to the volume set point. For example, preferably, thevolume of fluid dispensed through the nozzle is to be maintained at ±5%of the volume set point. Therefore, the maximum volume error value ischosen to be 7%. If the calculated volume error is within the acceptablelimit, the process of FIG. 6 is terminated. If the volume error exceedsthe maximum limit, the supervisor control 49 at 410 then computes theaverage value of the flow rate during the prior dispensing cycle asfollows: ##EQU7##

As described, with respect to the operation of the servo control 74 at214 of FIG. 2, at the end of a dispensing cycle, the microprocessor 76reads the value of the counter 114 and writes that value into a locationof dual port RAM 70. Therefore, the supervisor control 49 has availableto it a value representing the volume of fluid dispensed during theprevious dispensing cycle. In addition, in a manner similar to thatdescribed with respect to the calibration process, during the flowcontrol process of FIG. 5, the servo control 74 will start and stop atimer in response to detecting the respective ON and OFF states of thedispensing gun ON/OFF signal. Therefore, at the end of a dispensingcycle, that timer will have recorded the total dispense time, that is,the dispensing gun is ON during the dispensing cycle; and that totaltime is written by the servo control 74 into the dual port RAM 70. Thetotal dispense time is used by the supervisor control 49 in computingthe average flow rate at 410.

At 412, the supervisor control 49 executes a subroutine that computesthe change in flow rate in accordance with the following.

    ΔFR=FR.sub.n -FR.sub.n-1

where

FR_(n) =Average Flow Rate over Current Dispensing Cycle

FR_(n-1) =Average Flow Rate over Prior Dispensing Cycle

In order for a change of flow rate to be computed, at least twosuccessive dispensing cycles must have occurred. Therefore, therecompute flow characteristics subroutine must be executed twice insuccession; and during each execution, the process at 408 must determinethat the volume error exceeds the maximum value. During the firstiteration, the process at 410 will compute a first average flow rate forthe first dispensing cycle. During that iteration, the process at 412 isunable to compute a change in flow rate since only one average flow ratevalue exists. After the next dispensing cycle, if at 408 a volume erroris detected that exceeds the maximum error, the process at 410 will thencompute another average flow rate value for the immediately precedingdispensing cycle; and the process at 412 is then able to compute adifference or change in average flow rate values between two dispensingcycles in which there is an excessive volume error. Thereafter, at 414the process determines whether the change in average flow rate isgreater than a minimum change. If there is not a significant change inflow rate, the subsequent recalculation of the N value at 416 will notprovide a significantly different value, and therefore, will providelittle or no benefit. If the change in average flow rates is greaterthan the minimum limit as detected at 414, the process then at 416recalculates the value of the flow characteristic constant N as follows.##EQU8## where FR_(n) =Average Flow Rate over Current Dispensing Cycle

FR_(n-1) =Average Flow Rate over Prior Dispensing Cycle

P_(n) =Average Pressure over Current Dispensing Cycle

P_(n-1) =Average Pressure over Prior Dispensing Cycle

The average value of pressure during the prior dispensing cycle isdetermined at 230 in FIG. 3 as part of the integration of nozzlepressure values. The subroutine at 230 not only integrates the IPNvalue, but also integrates the P_(MEAS) value as follows:

    P.sub.AVG SUM =P.sub.AVG SUM +(P.sub.MEAS)

Therefore, at the end of a dispensing cycle, the process at 230 producesan average pressure value, that is calculated as follows: ##EQU9## whereCounter=Counter+1

There are applications where the fluid dispensing process may experiencechanges in temperature that produce variations in the flowcharacteristics of the fluid that are not satisfactorily accommodated bythe recalculation of the flow characteristic constants A and N. Forexample, a fluid dispensing cycle on a large workpiece may be of suchlength that temperature variations can adversely affect the flowcharacteristics of the fluid and the quality of the application of thefluid onto the workpiece. In those situations, a temperaturecompensation is desirable and is either programmably, or, manuallyselected; and that selection will be detected at 364 of FIG. 5; and atemperature compensation subroutine is executed at 368.

The temperature of the fluid being dispensed through the nozzle ismeasured by the temperature sensor 106 and is read by the microprocessor76 in executing the servo control process of FIG. 3. The temperature isread at 232 of FIG. 3, and the microprocessor 76 writes the value of thetemperature to an appropriate location in the dual port RAM 70. Inexecuting the subroutine at 368, the servo control 74 first detects achange of temperature of the fluid being dispensed between successiveiterations of the flow control loop of FIG. 5. There are manystatistical techniques for detecting and determining a change oftemperature to be used by the servo control 74. The change oftemperature may be based on an average of temperature readings atdifferent points in time. For example, the servo control loop of FIG. 3which reads the temperature at 232, iterates preferably every 2 ms.However, the flow control process of FIG. 5 iterates every 10 ms.Therefore, with each iteration of the flow control process, thesubroutine at 368 can monitor and average five different readings of thetemperature sensor. Further, the temperature compensation subroutine at358 may further utilize temperature readings that are based on movingaverages of the measured temperature. In addition, the subroutine 368may be designed so that a change in temperature is not recognized orprocessed until it exceeds a predetermined minimum magnitude. The exactprocess for detecting and establishing a particular temperature valueand the determination of a minimum change in temperature will depend onthe accuracy, resolution and repeatability of the temperaturemeasurement system, that is, the temperature sensor 106, the RTDinterface 102, and the D/A converter 88 of the temperature measurementsystem. The desired responsiveness of the model to the changes intemperature is also an important factor to consider. After the change intemperature is determined, the subroutine at 368 then recomputes thevalue of the b-term as follows. ##EQU10## where A_(n) =First FlowCharacteristic--Current Dispensing Cycle

A_(n-1) =First Flow Characteristic--Prior Dispensing Cycle

T_(n) =Temperature--Current Dispensing Cycle

T_(n-1) =Temperature--Prior Dispensing Cycle

Thereafter the desired pressure at 366 will be determined pursuant tothe following flow rate model. ##EQU11## where P_(DES) =Desired NozzlePressure

FR_(DES) =Desired Flow Rate

A=First Flow Characteristic Constant

N=Second Flow Characteristic Constant

ΔT=Measured Change in Temperature

b=Temperature Sensitivity Constant

An initial value for the temperature sensitivity constant b can bedetermined by dispensing fluid on successive workpieces and then usingthe measured temperature values and calculated values of the first flowcharacteristic constant A for each part to compute the value of b.

In use, the fluid dispensing control of the present invention providessubstantial advantages over previous controls. First, the previouscalibration cycle during which fluid was deposited on a workpiece inorder to empirically determine the volume set point is eliminated.Therefore, the dispensing control of the present invention is not in anyway dependent on workpiece related parameters. The control of thepresent invention further produces initial values of the flowcharacteristics of the fluid being dispensed by automatically executinga calibration cycle with as few as three and preferably four datapoints. Previously eight to ten data points were used to establish aflow rate to pressure relationship. Further, the dispensing controlproduces desired nozzle pressure values by evaluating a model of flowrate of the fluid through the nozzle as a function of the initial valuesof the flow characteristics and a desired flow rate value. That model isindependent of the volume set point and therefore is immune from anyinaccuracies or instabilities that may be introduced into the dispensingcontrol system because of inaccuracies in determining the volume setpoint. In addition, the value of the flow characteristic correlated tothe flow rate-pressure relationship is preferably updated everydispensing cycle thereby continuously adjusting the flow rate model forchanges in viscosity. A second flow characteristic constant within theflow rate model which is correlated to the flow non-linearitiesintroduced by shear effects of the fluid flowing through the nozzle isinitially determined in a calibration process and subsequently updatedin response to significant detected errors in the volume of fluid beingdispensed.

In a further unique aspect of the invention the fluid dispensing controlevaluates a flow rate model that includes temperature as a variable.

The control measures changes in temperature and reevaluates the model toproduce new desired nozzle pressure values thereby adjusting the flow offluid through the dispensing gun as a function of those changes intemperature.

While the invention has been set forth by a description of theembodiment in considerable detail, it is not intended to restrict or inany way limit the claims to such detail. Additional advantages andmodifications will readily appear to those who are skilled in the art.For example, even though the value of the flow characteristic constant Ais described as being recomputed during every dispensing cycle, in someapplications, the flow characteristic constant A may recomputed severaltimes during a fluid dispensing cycle; or alternatively, A may berecomputed only when the detected error in the volume of fluid dispensedexceeds a predetermined minimum value, that is, independent of anyparticular dispensing cycle. Further, the integration described hereinis generally a rectangular integration; however, a trapezoidalintegration or other integration schemes, as appropriate, may be used.While the preferred embodiment of the invention has been described inassociation with dispensing a bead of fluid or adhesive, the flowcontrol techniques can be used other fluid dispensing applications.

The invention therefore in its broadest aspects is not limited to thespecific details shown and described. Accordingly, departures may bemade from such details without departing from the spirit and scope ofthe invention.

What is claimed is:
 1. A method of automatically calibrating a systemfor dispensing a fluid through a nozzle onto a workpiece by determininga flow characteristic constant as a function of fluid flowcharacteristics for the fluid and nozzle, wherein the system includes aflow meter providing an output signal as a function of a volume of thefluid dispensed, the method comprising:dispensing fluid at differentflow rates during successive calibration dispensing cycles; measuringduring each calibration dispensing cycle a plurality of nozzle pressurevalues; calculating an average pressure value from the nozzle pressurevalues measured during each of the calibration dispensing cycles;determining a measured volume value representing the fluid dispensedduring each of the calibration dispensing cycles in response to outputsignals from the flow meter; calculating a flow characteristic constantusing the averages of the nozzle pressure values and the measuredvolumes of fluid dispensed during the calibration dispensing cycles. 2.The method of claim 1 further comprising:determining a plurality of datapoints, each of the data points being determined from an averagepressure value calculated during one of the calibration dispensing cycleand a corresponding measured volume value determined during the one ofthe calibration dispensing cycles; determining a linear relationshipwith respect to the plurality of data points; and determining the flowcharacteristic constant as a function of the linear relationship.
 3. Themethod of claim 1 further comprising:computing a natural log value ofeach of the average pressure values; computing a natural log value ofeach of the measured volume values; determining a plurality of datacoordinate values, each data coordinate value being determined by anatural log value of an average pressure value measured during one ofthe calibration dispensing cycles and a natural log value of a measuredvolume value measured during the one of the calibration dispensingcycles; and performing a linear regression on the plurality datacoordinate values to identify a straight line represented by the datacoordinate values.
 4. The method of claim 3 further comprisingperforming a least squares regression on the plurality of datacoordinate values.
 5. The method of claim 3 further comprisingdetermining a first flow characteristic constant as a function of aslope of the straight line.
 6. The method of claim 5 further comprisingsetting the first flow characteristic constant equal to the slope of thestraight line.
 7. The method of claim 3 further comprising determining asecond flow characteristic constant as a function of a y-intercept ofthe straight line.
 8. The method of claim 7 further comprisingdetermining the second flow in accordance with the following:

    A=ε.sup.y intercept


9. The method of claim 3 further comprising measuring a length of timefor each of the calibration dispensing cycles.
 10. The method of claim 9further comprising determining the natural log of each of the measuredvolume values in accordance with ##EQU12## where VOL_(MEAS) @X%=Measured Volume at the Set % of Maximum Pressure.
 11. The method ofclaim 1 further comprising dispensing fluid during successivecalibration dispensing cycles at different flow rates extendingsubstantially over the whole range of flow rates.
 12. The method ofclaim 11 further comprising:dispensing fluid during a first calibrationdispensing cycle at approximately 100% of a maximum flow rate; anddispensing fluid during a second calibration dispensing cycle atapproximately 10% of the maximum flow rate.
 13. The method of claim 12further comprising dispensing fluid during a third calibrationdispensing cycle at approximately 50% of a maximum flow rate.
 14. Amethod of automatically calibrating a system for dispensing a fluidthrough a nozzle onto a workpiece by determining a flow characteristicconstant as a function of fluid flow characteristics for the fluid andnozzle, wherein the system includes a flow meter providing an outputsignal as a function of a volume of the fluid dispensed, the methodcomprising:dispensing fluid at different flow rates during successivecalibration dispensing cycles; measuring during each calibrationdispensing cycle a nozzle pressure value; determining a measured volumevalue representing the fluid dispensed during each of the calibrationdispensing cycles in response to output signals from the flow meter;determining a plurality of data points, each of the data points beingdetermined from an pressure value determined during one of thecalibration dispensing cycles and a corresponding measured volume valuedetermined during the one of the calibration dispensing cycles;determining a linear relationship with respect to the plurality of datapoints; and calculating a flow characteristic constant as a function ofthe linear relationship.
 15. The method of claim 14 further comprisingidentifying a straight line represented by the plurality of datacoordinate values.
 16. The method of claim 15 further comprisingdetermining a first flow characteristic constant as a function of aslope of the straight line.
 17. The method of claim 14 furthercomprising setting the first flow characteristic constant equal to theslope of the straight line.
 18. The method of claim 17 furthercomprising determining a second flow characteristic constant as afunction of a y-intercept of the straight line.
 19. The method of claim18 further comprising determining the second flow in accordance with thefollowing:

    A=ε.sup.y intercept.


20. 20. A method of automatically calibrating a system for dispensing afluid through a nozzle onto a workpiece by determining fluid flowcharacteristics for the fluid and nozzle, wherein the system includes aflow meter providing an output signal as a function of units of volumeof the fluid dispensed, the method comprising:dispensing fluid at afirst flow rate during a first calibration dispensing cycle;periodically measuring during the first calibration dispensing cycle anozzle pressure to provide a plurality of nozzle pressure values;calculating an average of the nozzle pressure values measured during thefirst calibration dispensing cycle; determining a measured volume offluid dispensed during the first calibration dispensing cycle inresponse to the output signal from the flow meter; iterating the abovesteps of dispensing measuring, calculating and determining to dispensefluid at a second flow rate during a second calibration dispensingcycle; and calculating a flow characteristic constant using the averagesof the nozzle pressure values and the measured volumes of fluiddispensed during the first and second calibration dispensing cycles. 21.A method of automatically recalculating a flow characteristic constantfor a fluid dispensing system dispensing a fluid through a nozzle onto aworkpiece and including a flow meter providing an output signal as afunction of a volume of the fluid dispensed, the flow characteristicconstant being determined as a function of fluid flow characteristicsfor the fluid and the nozzle, the method comprising:dispensing fluidthrough the nozzle during a dispensing period; measuring during thedispensing period a plurality of nozzle pressure values; determining ameasured volume value representing the fluid dispensed during thedispensing period in response to output signals from the flow meter; andcalculating a flow characteristic constant in response to the pluralityof nozzle pressure values and the measured volume value during thedispensing period.
 22. The method of claim 21 further comprisingintegrating the plurality of nozzle pressure values over the dispensingperiod.
 23. The method of claim 22 wherein the first flow characteristicconstant is determined in accordance with ##EQU13## where VOL_(MEAS)=Measured Volume of Fluid Dispensedk=Scaling Factorand ##EQU14## whereP_(MEAS) =Measured Pressure During Dispensing Period N=Second FlowCharacteristic Constant start=Start of Dispensing Period end=End ofDispensing Period.